The digital substation architecture consists of 3 levels.
- The first is the process level, interfacing with the primary equipment in the switchyard.
- Second is the protection and control level, including IEDs traditionally called “secondary equipment” (protection, measurement devices, bay controller, recorders, etc.).
- Third, the station control level handles communication within the substation and control system, coordination with the substation operational functions and station-level support.
Real-time operational measurements and other data are polled from sensors embedded within the primary system and communicated to the devices that must act on those measurements via a process bus. Smart devices and systems within the substation can immediately process this data. By subscribing as clients to this data flow over an Ethernet process bus, the information from the power system is distributed and communicated much more efficiently to the bay level than in conventional hardwired schemes. The DS Agile architecture can be set up as a self-healing ring or as an IEC 62439 parallel redundancy protocol (PRP) redundant star, with numerous advantages as summarized by Richards’ colleague, Denis Chatrefou, Digital Substation R&D Coordinator. “The extensive self-diagnosis capability of digital devices maximises the up-time of the substation. Any degradation in the performance of an asset is pinpointed in real time. Inherent redundancy in the system can be employed to self-heal the operation, which permits troubleshooting without the need for any primary system outage.”
A typical DS Agile digital control system (DCS) incorporates a range of IEDs, such as protection relays, measurement devices and monitoring equipment, as well as bay controllers, Ethernet switches and gateways.
The network can be local to the substation or can interconnect several dispersed substations, and can be linked to the grid control room. The process bus is the link by which the primary equipment information travels back to the substation’s relay room.
In a fully digital architecture, control commands (switchgear operator commands, protection trips) are also routed to the primary devices via the process bus, in the opposite direction. In a digital substation, thanks to IEC 61850, the IEDs interact with the field via the process bus, and with other peer devices in the bay, other bays, and the digital control system via the station bus. In the digital substation, the station bus is much more than a traditional SCADA bus. It permits many devices to exchange data, supports peer-to-peer device communication, and links to wide-area control units (WACUs) for wide-area communication between substations or between voltage levels.
The BRIC economies (Brazil, Russia, India, China) are building or planning large power plants far from their main population and industrial centres, and wide-area networks are developing in other markets too, with several countries in Europe planning large offshore wind farms, for instance. This is placing an increasing strain on transmission corridors, and operators may find themselves working ever closer to the limits of the networks.
To ensure system stability over these long distances and to prevent the propagation of disturbances requires real-time monitoring and the timely implementation of countermeasures. Phasor measurement units (PMU) (1) are at the heart of wide-area monitoring, protection and control. They collect highly time-synchronised data on system parameters such as voltage, current and frequency, which they then report to the PDC (phasor data concentrator) for visualisation, data storage and, most importantly, for running a variety of on-line and off-line applications in systems analysis and control. This enables operators to optimize the scheduling of transmission capacity, to respond to contingencies, and to preserve stability after disturbances.
The Alstom P847 PMU exceeds the requirements of IEEE C37.118 Level 1 with enhanced performance under off-nominal and dynamic system conditions by means of an advanced frequency-tracking algorithm.
(1) More on our article “Synchrophasor solutions” .